Up to now, as the clutch of this type, there is an electromagnetic clutch for transmitting a rotational driving force to a car air conditioning compressor from an external power source such as an engine through a belt (for example, refer to Patent Document 1).
In the above electromagnetic clutch, an electromagnetic coil of a stator is energized to form a magnetic circuit in which a magnetism passes through a rotor and an armature of a hub. With this configuration, the armature of the hub is attracted to the rotor by the aid of an electromagnetic force generated by the magnetic circuit. For that reason, the armature and the rotor come into contact with each other, and a friction occurs on a contact surface (hereinafter referred to as “friction surface”) between the armature and the rotor. For that reason, the rotor, the armature, and a rotary shaft are rotated together by the aid of the driving force transmitted from the external power source through the belt. In other words, the clutch is turned on, and the driving force to be transmitted from the external power source through the belt is transmitted to the car air conditioning compressor through the rotor and the hub.
On the other hand, the energization of the electromagnetic coil of the stator is stopped to vanish the electromagnetic force. For that reason, the armature is separated from the rotor by the aid of a restoring force of a rubber member disposed between an outer metal fitting and an inner metal fitting configuring the hub. In other words, the clutch is turned off, and transmitting of the power to the car air conditioning compressor from the external power source through the rotor and the hub is stopped.
In recent years, for the purpose of improving a fuel efficiency, a reduction in size and weight of the electromagnetic clutch has been required by vehicle manufactures, resulting in requirements of a reduction in an axial dimension of the electromagnetic clutch and a reduction in a radial dimension of the rotor. In that case, abrasion powder generated due to the abrasion of the friction surface of the rotor and the friction surface of the armature may remain and be unlikely to be discharged.